


A Father's Pride

by SoloShikigami



Category: Undertale (Video Game)
Genre: Incest, Mating Cycles/In Heat, Multi
Language: English
Status: Completed
Published: 2019-12-12
Updated: 2019-12-12
Packaged: 2021-02-26 04:08:20
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,867
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21767365
Author URL: https://archiveofourown.org/users/SoloShikigami/pseuds/SoloShikigami
Summary: Sans isn't feeling so well one night, and Papyrus seems to grow more protective of his brother than he should. Gaster sees what's going on with his creations and guides them through their first experience with a monster heat cycle.
Relationships: W. D. Gaster/Papyrus/Sans
Comments: 8
Kudos: 79





	A Father's Pride

**Author's Note:**

> This fic was requested by Ganzooky, and they made a cover for it! Find it and their awesome art here - https://twitter.com/ganzooky/status/1072057030335025152?s=20

Gaster felt like he had to be one of the most narcissistic monsters in all the Underground. He knew he was too proud of himself, too proud of his creations to be otherwise. He was sitting at the kitchen table, his eyes casually travelling from the skeleton sitting across from him making notes and calculations on a blueprint to the skeleton humming as he made dinner and cleaned the kitchen.

They had turned out to be much more than Gaster had planned. They were meant to be mere experiments to study the nature of souls; he didn't expect them to have intellects, minds of their own, and be fully-functioning monsters.

Sans had shown signs that he would do very well as an assistant, so Gaster began bringing home books and papers for Sans to study, and soon moved on to bringing part of his work home for Sans to go through. He didn't dare take him to the lab just yet, if ever.

Papyrus had intelligence in his own rights, but lab work seemed to bore him; he seemed much happier attending to the other two, ensuring their home was clean, keeping an eye on Sans as he studied and worked, and making sure they were fed and well cared for.

They served Gaster well and were a happy little family.

"FATHER! SANS! DINNER WILL BE READY MOMENTARILY! PLEASE WRAP UP YOUR WORK AND CLEAN UP!" Papyrus said boisterously as he checked the oven.

"Thank you, Papyrus," Gaster said, stretching his back. "Come along, Sans."

He started to gather his papers, and after a moment he noticed that Sans didn't move. This was truly an odd event; Sans would work hard when set to a task, however, he leapt at any chance to abandon his work for a break. Upon a closer inspection, he noticed that Sans hadn't been working much, either. His hands looked almost plastered on the side of his head, his eyes were darting nervously about, and... Was it Gaster, or did his creation have a certain odor about him?

"Sans? Are you all right?" Gaster asked.

"I, I don't feel so good," Sans muttered.

"What is troubling you?"

"I, uh, I dunno, just, like, nervous, and um... Warm, but cold? I feel..." Sans looked pained as he struggled for the right word. "Empty? Yeah, I feel empty."

"I see."

' _This will be interesting, if it's what I suspect,_ ' Gaster thought.

"Is Sans all right, Father?" Papyrus asked, standing by the table with his arms full of flatware and his face full of concern.

"I think he needs a break and a good meal," Gaster said, reaching for the papers so Papyrus could set the table. "No more work, tonight."

"Oh! Can we use the television after dinner?" Papyrus asked.

Gaster had found an old entertainment setup and a box of black rectangular objects called video cassettes. They had watched a few of them and to watch the tapes was a rare treat.

"I think it is warranted this evening; is that agreeable to you, Sans?" Gaster asked.

"Yeah, sure," Sans murmured.

All through the meal, Gaster kept a close eye on both Sans and Papyrus. Papyrus asked questions about what he and Sans were working on, spoke about the puzzles he had invented, but as hard as he was trying to keep up a positive attitude, he constantly gave his brother a worried glance. Those looks increased and he became quieter as the meal drew to its close. Sans hadn't eaten much; he was silent throughout the meal and he seemed to be jumpy and agitated.

"Sans, go and change your clothes; it may make you feel better," Gaster said.

Sans nodded and slowly left the table.

"Is he going to be all right, Father? Should I go help him?" Papyrus asked, wringing his hands nervously.

Gaster took note of Papyrus' behavior. "I'm sure he can handle changing his own clothing, Papyrus; he isn't a child. Neither are you."

Papyrus sighed and stood to begin clearing the table.

"I know," he said, his voice dropping into an even softer tone. "But I'm worried."

"Why? Why should Sans concern you so?"

"Well, we are, what? Brothers? That is the easiest word to use, but it doesn't encompass it all, does it?"

"Explain."

Papyrus stilled before reaching for Gaster's plate. "I want to protect him," he glanced over at Gaster. "Whatever is wrong with him, whatever he is feeling, I want to fix it."

Papyrus' hands began to shake, when he realized it, he brought them closer to his face, as if he couldn't believe they were doing so. After a moment, he clenched them into fists, stilling them only slightly. He glanced over at Gaster and the scientist was taken aback by the look in Papyrus' usually sweet and kind features. He looked almost angry, a slight bit of pale orange magic swirling in his right eye.

"I need to protect him, father, I _must_ protect Sans. He’s mine."

' _Interesting, the dynamic wasn't what I expected for their first,_ ' Gaster thought to himself.

"You will, Papyrus, but take it easy; you don't want to frighten your brother," Gaster said.

Papyrus blinked and tilted his head, behaving as if the past minute never happened.

"I could never frighten Sans, he is quite brave, after all."

Papyrus returned to his cleaning while Gaster continued to ponder.

~~~~~

The movie they had chosen to watch seemed to be of the romantic variety, much to Gaster’s amusement, but his focus stayed on his boys. Papyrus kept glancing over at Sans with a worried frown while Sans fidgeted during most of the movie. Gaster sat on one side of the couch, leaning on the arm. Sans always sat in the middle, usually slouched with his hands in his pockets, and Papyrus sat on the other side, his legs crossed on the cushion.

They came to a part of the movie where the main characters were getting intimate when Sans leaned forward slightly with a low moan.

"G-Gaster, this is getting worse," Sans murmured. "Please, what's happening to me?"

"Sans?" Papyrus turned to him. "Are you all right?"

Sans shook his head. Papyrus immediately reached out to him, and though there was a split second where Sans' initial reaction was to go towards him, he suddenly backed up into Gaster, who put his hands on his shoulders.

"Sans?" Papyrus murmured, reaching a hand towards him.

Sans pressed his back against Gaster. "I, I can't, I want but can't," he stammered.

"Sans," Gaster said firmly, emphasizing his words by squeezing the young skeleton's shoulders. "Speak."

"It’s so hot, my soul is on fire, my bones are on fire, I feel so _empty_ ," he looked up at Gaster miserably. "I, I'm going to die if this doesn't stop."

"Father! Please! You're a doctor, do something!" Papyrus said frantically, caught between his desire to reach out to his brother and keeping his hands to himself out of respect.

"Boys, relax," Gaster said, rubbing Sans' arms soothingly.

Sans moaned in response, then blushed at his own reaction.

"This is unexpected, but quite natural. Sans, you are going into your first receptive heat. Papyrus, you are responding to it."

"What does that mean? Are we both ill? I don't feel sick," Papyrus said, looking over himself for signs of ailment.

"No, again, it’s all natural for monsters your ages," Gaster said. "Sans, your body is telling you that it is ready to reproduce."

"What?! I'm male, and I don't want kids!" Sans twisted and flailed, trying to get out of Gaster's grip, but he was firm.

"Sans, please, calm yourself," Gaster sighed.

Sans whimpered pathetically, Papyrus let out a small whine, one hand pressing over his sternum, still hesitating to reach towards Sans.

"We are made almost completely of magic. We can form our reproductive organs mostly at will and we are not constrained by who we mate with," Gaster explained.

Sans shuddered and groaned as a wave of heat washed over him. Gaster sighed, stroking Sans' skull.

"The rest of the lesson must be more practical in nature," Gaster murmured. "Sans, don't worry about reproducing, it won’t happen right now. For now, we should relieve some of your tension."

"Do something," Sans groaned.

"Papyrus, this is where you come in," Gaster said, beckoning the other one to come closer and Papyrus eagerly scooted forward. "Sans’ heat seems to have triggered yours. An alpha heat is likely the best way to explain it."

"Alpha?" Papyrus asked, tilting his head.

"Your drive is to protect and impregnate your mate. We'll discuss that last bit later. For now, your brother needs you."

"What do I do?" Papyrus asked.

"Do what comes naturally to you."

Papyrus glanced up at Gaster, still unsure, but Gaster gave him an encouraging smile. He looked back at Sans, who still looked like a frightened animal. Papyrus raised a hand slowly and brushed it down the side of Sans' face. Sans gasped a little at the contact, but he leaned into the touch regardless.

"It's okay, Sans," Papyrus murmured. "I'm going to make it okay."

"Papyrus," Sans whispered. "It hurts, it burns..."

"Shh, shh, it's okay," Papyrus hushed him.

Papyrus leaned in slowly, their teeth meeting with a soft _clack!_ and they both sighed as if being relieved of a great pain.

Gaster sat back and watched the two as they explored their first kiss, both fascinated and a little ashamed at being witness to such an intimate display.

Sans leaned into Papyrus, feeling a pull towards the other that he never remembered feeling before. His body and soul yearned to touch and be touched, to bring in this other being into himself so he could be whole again.

"Mmm, wait," Papyrus pulled away, making Sans whine. "Wait, Sans, my mouth feels funny."

"Feels funny how?" Gaster asked.

"Like, something inside," Papyrus said, his jaw working as he tried to describe the sensation.

Gaster sat up. "Open.”

Papyrus obeyed the order. Gaster reached forward, cupping his jaw as his thumb pressed the orange appendage in his mouth.

“That, my dear, is a tongue. Let me show you what you can do with it,” Gaster said.

Papyrus leaned forward and Gaster couldn’t help chuckling at his eagerness. Just before their mouths met, Gaster reached out with his tongue and wrapped it around Papyrus’, drawing a surprised sound. He hummed softly in appreciation as they parted, stroking Papyrus’ cheekbone.

“Do you know what to do?” Gaster asked.

“Yes, Father,” Papyrus said with a smile, leaning into the caress before returning to Sans, who was now reaching for him, desperate again for contact.

Gaster went back to observing. Sans’ body seemed to follow Papyrus’ lead, as he could see a bit of blue magic slipping between teeth as they kissed. Papyrus moved a little to the side so he could press his tongue along Sans’ neck, exploring and testing, finding the spots that made Sans moan and squirm and being sure to lavish the most attention there.

While Sans seemed to enjoy the attention, he was also getting impatient. He reached out and took one of Papyrus’ hands and pressed it between his legs.

“Oh, oh my,” Papyrus murmured. “It’s so hot down there, and… damp?”

“You better remove them,” Gaster said. “I will help Sans out of his shirt, and you might as well undress, too.”

Papyrus obeyed, then he nearly tore his shirt off. “What about you, Father?”

Gaster smiled as he eased the t-shirt over Sans’ head. “I am here to observe and guide you.”

Sans panted. “Do something, please, it burns,” he murmured.

Papyrus’ eyes widened as he saw the mound of pulsing magic that formed over and around Sans’ pelvic inlet. He saw a darker blue magic dripping from it and he reached forward t touch it before it fell on the couch cushions.

“Oh, my, Sans,” Papyrus said. “May I touch you?”

“Please, oh, stars, please, Papyrus,” Sans begged, leaning back against Gaster again, who wrapped his arms lightly around his ribcage to add some assurance and stability to the small skeleton.

Papyrus’ fingers brushed between the slit in the mound of magic and Sans jerked suddenly.

“I’m sorry!” Papyrus cried, pulling his hand back.

“It’s all right, Papyrus,” Gaster said, stroking down Sans’ ribcage to soothe him. “Sans would say something if it didn’t feel good.”

“Pap, please, please,” Sans begged again.

Papyrus moved his hand into him again, drawing out another jerk but also a moan of pleasure. Papyrus stroked along the folds of magic, exploring again and learning things like if he brushed over a certain nub Sans would cry out in pleasure. He also found the source of the dripping magic and he eased a finger inside the hole gently.

“Yes, yes, Papyrus, please, fill me, please, I feel so empty,” Sans whined.

Papyrus nodded and slowly pressed two fingers inside of him, pumping them gently, waiting patiently for Sans to calm down again before he added a third.

“More, more,” Sans moaned.

“What do I do now?” Papyrus asked.

Gaster glanced down Papyrus’ body. “Hmm, I think your magic had responded well enough to give you something else to work with.”

Papyrus followed his gaze to his own pelvis and found his magic had created a rod, standing erect and almost as proud as Papyrus was. The young skeleton reached down with one hand and touched it carefully, then gripped it firmly and stroked.

“Oh, oh, my,” he murmured. “T-that feels nice.”

“Papyrus,” Sans moaned, as if to remind him that he was still there.

“Sans,” Papyrus murmured, moving himself so he was kneeling between Sans’ femurs, his hand still stroking himself, the other hand touching Sans again, his thumb rubbing gentle circles on the sensitive nub in Sans’ magic.

“Papyrus, please, I want you, I need you,” Sans implored.

“Do you?” Papyrus said, leaning over him and pressing a kiss to the side of his neck. “Tell me what you would like, Sans.”

“I, I need that, I need it inside now, please, I’m going to die otherwise.”

Papyrus smiled and leaned up to kiss Sans as he moved in closer, rubbing the orange head along blue folds before pressing carefully inside.

Sans cried out at the intrusion and Papyrus stopped but did not back away. He waited.

“More, more, please,” Sans begged breathlessly after a moment.

“Gentle,” Gaster murmured encouragingly.

Papyrus pressed in again, still slow, and while Sans panted and gasped, he didn’t seem to be in any pain. Soon, Papyrus’ pelvis was flush with Sans’. Papyrus was panting himself; Sans squeezed him tightly, he was burning hot, but it felt incredible.

“So good,” Sans said, wrapping his arms around Papyrus’ neck and pulling him in for a kiss. “Please move, when you can,” he murmured.

Papyrus nodded, pulling out only a little before pressing back in. Sans moaned again, so Papyrus moved again.

Their pace was slow and gentle, Sans’ head reeling from the sensation of being filled as Papyrus enjoyed the feeling of being surrounded. Their souls glowed slightly under their ribcages as they moved towards their crests.

Gaster observed it all with a mix of curiosity, wonder, and awe. The scientist in him kept mental notes of it all, the parent in him was near to bursting with joy in how well his sons were performing. He felt Papyrus’ gentle thrusts as Sans was pressed into him with each stroke, he could feel Sans’ pleased shudders and his ears drank in their moans of desire.

“W-wh-what, I, this feeling,” Sans suddenly moaned, a touch of panic in his voice.

“It’s all right,” Gaster assured him. “You are approaching the peak of your pleasure. Relax and let it go, your heat will be satisfied for a while once you reach it.”

Papyrus was approaching his as well, and moved a little faster to get there. Gaster watched as Papyrus’ body began to emit a cloud of orange magic that surrounded them and Sans’ was doing the same.

“P-P-Papyrus!” Sans cried out, gripping his brother tightly as his body tensed.

“Sans, Sans, oh stars, Sans,” Papyrus murmured, feeling the pressure in his groin building as well.

They both held each other tightly as their peaks hit, crying each other’s names. Gaster felt their combined magic press in on them all and he could feel a passing sense of hat his sons were experiencing. All at once, it seemed, it all dropped. Papyrus collapsed on top of Sans, his energy drained. Sans shook underneath him, wrapping his legs around his hipbones and holding him tight. Gaster kissed the back of Sans’ skull and reached down to stroke Papyrus’.

“You two are beautiful,” Gaster murmured. “Absolutely stunning; a father could not be more proud.”

He could feel that they acknowledge his presence and words, but both were too spent to reply. Gaster smiled, reaching for a blanket that was folded over the back of the couch and pulled it over them all. He knew that once Papyrus got his senses back, he would be devastated at the mess. He hoped that Sans would be well enough to eat.

For now, though, they needed rest.

**Author's Note:**

> Like what you read? Please check out my social media for more fun!  
> [ My Tumblr](https://soloshikigami.tumblr.com/)  
> [ My Twitter](https://twitter.com/SoloShikigami)


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